1. Field of the Invention
The present invention relates to a skutterudite thermoelectric material for directly converting heat to electricity caused by the Seebeck effect, to a thermoelectric couple using the thermoelectric material, and to a method of producing the thermoelectric material and the thermoelectric couple.
2. Description of the Related Art
Conventionally, Bi.sub.2 Te.sub.3 -based thermoelectric materials have been well known as thermoelectric materials based on the Seebeck effect, and have been used practically in some applications. However, the materials have a very narrow range of operating temperatures, and are limited to be used at around room temperature.
Furthermore, it is also known that such a thermoelectric material is formed of a CoSb.sub.3 -based intermetallic compound having a skutterudite crystal structure. This CoSb.sub.3 -based compound features high electron and hole mobility, and is expected to become a material wherein high thermoelectric conversion characteristics are compatible with a wide operating temperature range.
An important characteristic required for thermoelectric materials is represented by a figure of merit Z=S.sup.2.sigma./.kappa., wherein S is a Seebeck coefficient, .sigma. is an electric conductivity, and .kappa. is a thermal conductivity. Both higher Seebeck coefficient S and electric conductivity .sigma. in the thermoelectric material, and lower thermal conductivity .kappa. are desired to increase the figure of merit Z. These parameters are determined depending on the kinds and amounts of impurities added to the main ingredients of the thermoelectric material. In addition, the parameters are also changeable due to dispersing impurity particles in the crystal grain boundaries as a second phase
As a prior art, Japanese Patent Publication JP-A 9-260729 discloses an electric conductivity .sigma. of the sintered material being improved at the presence of a metal Sb phase as a second phase in the grain boundaries of a sintered material including CoSb.sub.3 as the main ingredient.
Furthermore, the above-mentioned Japanese Patent Publication JP-A 9-260729 also discloses a method of producing a thermoelectric sintered body. In this method, powder of a CoSb.sub.3 -based alloy including a metal Sb phase is formed under pressure, and heated at a higher temperature than the liquidus temperature of Sb, then, obtaining a sintered material in which the Sb phase dispersed in the grain boundaries.
Recently, compounds having filled-skutterudite structures represented by the formula of LnT.sub.4 Pn.sub.12 (Ln: a rare earth metal, T: a transition metal, and Pn: an element, such as P, As, Sb or the like) have received attention as thermoelectric materials.
Filled-skutterudite is a crystal wherein part of a pair of vacancies existing at the octant of the unit cell of the skutterudite crystal is filled with a heavy element, such as a rare earth metal. When the vacancies in the skutterudite crystal are filled with the atoms of a rare earth metal, such as Ce, the atoms of Ce oscillate because of a weak bond to Sb and act as the centers of phonon scattering. As a result, it can be expected that the thermal conductivity can be decreased significantly.
As a prior art regarding this, for example, D. T. Morelli and G. P. Meisner; "High figure of merit in Ce-filled-skutterudite," I.E.E.E., 15th International Conference on Thermoelectronics (1996), pp91, have studied the characteristics of CeFe.sub.x Co.sub.4-x Sb.sub.12 -based skutterudite thermoelectric materials filled with Ce and part of Co of which is substituted with Fe. According to this study, as the number x of substitutions of Fe with respect to Co increases in the range of 2 to 4, the electric conductivity increases, the thermal conductivity decreases, and the Seebeck coefficient decreases. To obtain the highest figure of merit, the number x of substitutions is required to have a suitable range.
In order to use these materials for a thermoelectric module, it is advantageous to form a p-n junction by using two kinds of Co--Sb-based filled-skutterudite thermoelectric materials. In this case, the filled-skutterudite structure filled with a rare earth metal can be used as a p-type thermoelectric material.
Regarding a thermoelectric module formed from thermoelectric materials, Japanese Patent Publication JP-A 9-64422 discloses that a Co--Pt--Sb-based compound is used as an n-type and a PbTe-based compound is used as an n-type, and that the two compound materials are joined to each other directly or indirectly via a metal conductor to form a p-n junction. In this method, the powders of the two compounds are press-formed integrally into a horseshoe-shaped compact so that they are joined to each other at their leading ends. The compact is then sintered to produce a thermoelectric couple.
In the case of a conventional material wherein a Sb phase is dispersed as a second phase to CoSb.sub.3, although the electric conductivity .sigma. increases, the Seebeck coefficient S decreases, whereby a power factor S.sup.2.sigma. is not improved significantly. In addition, the dispersion of the Sb phase does not decrease the thermal conductivity .kappa.. Therefore, the thermal conductivity must be decreased in order to further improve the characteristics of the CoSb.sub.3 -based thermoelectric material. In addition, depending on the grain size of the Sb phase to be mixed, dispersion becomes non-uniform, thereby causing segregation and unstable characteristics because of the segregation.
Although the filled-skutterudite structure is expected to have excellent thermoelectric characteristics, it is unstable thermally and is apt to be decomposed. In order that one vacant octant is filled with the heavy atoms of a rare earth element, the charges in a cell must be balanced. For example, in the case when a tetravalent atom of lanthanide is used for filling, charges can be compensated for by substituting four divalent Co atoms in the skutterudite structure with, for example, four trivalent Fe atoms. Since the skutterudite structure cannot be obtained by using only the Fe and Sb, part of the filled-skutterudite structure may be decomposed into FeSb.sub.2 and Sb, when the number of valencies of an element filled in the crystal lattices is not constant. In addition, in order to act as the centers of phonons, the filling element is required to have a weak bond to Sb in the lattices. For these reasons, the bond between the Ln and Sb atoms is thermally unstable. At high temperatures, the filling element falls from the lattice of the filling element, causing a problem of generating decomposition of skutterudite.
Furthermore, in order to assemble these materials so as to be used as a thermoelectric module, it is necessary to form a p-n junction by using two kinds of thermoelectric materials. Conventionally, regarding an n-type, it is reported that a material capable of having a high Seebeck coefficient S and a high electric conductivity .sigma. is obtained by adding Pd or Pt to the CoSb.sub.3 crystals (see Japanese Patent Publication JP-A 8-186294). However, to obtain a p-type thermoelectric material, it is considered that a metal element, such as an iron-group transition metal, i.e., Mn, Cr, Fe, Ru or the like, is added to the CoSb.sub.3 crystals. However, since the addition of such a metal abruptly decreases the Seebeck coefficient S, the power factor is not improved. For this reason, the characteristics of such a conventional thermoelectric material are unsatisfactory as a p-type material.
Moreover, the filled-skutterudite thermoelectric material having a filled-skutterudite structure filled with rare earth metals is a p-type. Regarding an n-type, a method of controlling the carrier level by adding Co has been attempted. However, satisfactory results have not yet been obtained.
Additionally, in the case when materials used at high temperatures, such as the skutterudite thermoelectric materials, is formed into a module, a brazing method or the like has been used to obtain a p-n junction at the high-temperature end of the module. In this case, however, problems of improper contacts or wiring breakage are caused at increased usage temperatures due to the difference between the thermal expansion coefficient of a metal used for connection and that of the thermoelectric material.